1. Field of the Invention
This invention relates to a circumferential flow type dynamic pressure bearing having opposed surfaces formed with proximity portions that generate a support force by gas pressure to thereby carry out a high-speed rotation supporting operation. More particularly, the invention relates to a circumferential flow type dynamic pressure bearing of this kind which has grooves formed therein to secure compatibility with high-humidity environmental conditions, and is at the same time capable of reducing noise generated by vibration caused by the run-out of the grooves.
2. Description of the Related Art
The present applicant proposed a dynamic pressure bearing compatible with high-humidity environmental conditions in Japanese Unexamined Patent Publication (Kokai) No. Hei 9-210052. The bearing disclosed therein includes a circumferential flow type dynamic pressure bearing for use in a rotary mirror scanner using a rotary mirror operating at a high rotational speed. This bearing is made compatible with high-humidity environmental conditions while preserving the run-out precision.
As shown in FIG. 10, this dynamic pressure bearing 101 is comprised of a dynamic pressure shaft 102 and a dynamic pressure sleeve 103 fitted thereon. The dynamic pressure shaft 102 has opposed surfaces formed with proximity portions 104, and the dynamic pressure sleeve 103 has a groove 105 serving as a water drain passage.
The proximity portions 104 are portions formed to provide reduced clearance between the opposed surfaces of the shaft 102 and the sleeve 103, whereby the support force of gas pressure is generated by the relative rotation of the shaft 102 and the sleeve 103 with respect to each other without producing a flow of the gas in the axial direction. This prevents foreign matter from being drawn in and thereby enables the bearing to maintain the precision over a long period of time. Further, the groove 105 prevents moisture vapor from being liquefied by the change in gas pressure so that it is possible to prevent a water layer from forming without requiring the flow of gas in the axial direction. Thereby, abnormal loads are prevented from being applied to the bearing due to the water layer under high-humidity environmental conditions.
The dynamic pressure shaft 102 of the dynamic pressure bearing 101, however, undergoes run-out or radial deviation of the axis thereof due to an imbalance which occurs whenever each proximity portion 104 passes by the groove 105 during rotation of the shaft 102. If the shaft 102 has a triple-arc-shaped pressure-generating surface, it undergoes three radial deviations per rotation of the shaft. Hence, it generates vibrations having a frequency three times as high as the rotational speed of the shaft 102. Due to this vibration, problems are caused in that temporary audible sounds are produced in the course of change in the rotational speed before the operating rotational speed of the bearing is reached. This problem of noise generated by vibration caused by radial deviations of the shaft has not been solved even by forming the groove into a helical shape.